In general, a battery-driven electrical device has a function of notifying a user of an approximate operation period that the electrical device can operate for using the battery mounted therein.
Various propositions have been made in relation to the function. For example, a battery pack described below is known. The object of the battery pack is to allow a user of a device to more correctly recognize the display of the remaining capacity of a rechargeable battery. When it is detected that an order electric current has dropped to a predetermined value during charging, a cumulative value of the current indicating the battery capacity is reset to a reference cumulative value. Furthermore, the reference cumulative value and a threshold value corresponding to the remaining capacity of the battery of zero are corrected according to a degradation level based on the number of times the battery is charged and the like.
In recent years, there is increased awareness of environmental problems and energy saving. In light of this increased awareness, there is a focus on buildings such as housing and offices that are designed to fully exploit natural energy, referred to as so-called renewable energy, such as solar light, solar heat, and wind power, instead of limited fossil fuels. Such buildings attempt to reduce the environmental burden of indoor life as much as possible. Of the types of renewable energy, especially solar photo-voltaic generation is becoming commonplace in familiar settings. Furthermore, housing designed to comprehensively promote a reduction in the environmental burden for the entire building (eco-housing) is proposed.
When performing power generation using natural energy, starting with photo-voltaic generation, the amount of power generation fluctuates greatly depending on natural conditions such as the season, the time of day, and the region. Tendencies of the fluctuation in the amount of power generation do not match those of the fluctuation in the amount of power consumption. A power management system provided with a storage battery in which surplus energy is stored is proposed from the perspective of effectively using the electrical energy that is generated. The power management system stores the surplus electrical energy that is generated using photo-voltaic generation during the daytime in the storage battery, for example. After sunset, savings are made in the power consumed from the power system by performing independent operation in which the energy that is stored in the storage battery is supplied to the electrical devices without receiving a supply of power from the power system. When there is a power outage in the power system, it is possible to drive the electrical devices using the energy stored in the storage battery.